Effects of growth hormone administration on fuel oxidation and thyroid function in normal man

Abstract

In a randomized, double-blind, placebo-controlled, cross-over study, we examined the effects of 14 days of growth hormone (GH) administration (12 IU/d subcutaneously) on energy expenditure (EE), respiratory exchange ratio (RER), and thyroid function in 14 normal adults of normal weight (eight men and six women). EE (kcal/24 h) was significantly elevated after GH administration (2,073 ± 392, [GH], 1,900 ± 310, [placebo], P = .01). RER was significantly lowered during GH administration (0.73 ± 0.04 v 0.78 ± 0.06, P = .02), reflecting increased oxidation of lipids. Total triiodothyronine (TT 3) (nmol/L) and free T 3 (FT 3) (pmol/L) increased significantly during GH (TT 3: 1.73 ± 0.06 [GH], 1.48 ± 0.08 [placebo], P = .01; FT 3: 6.19 ± 0.56 [GH], 5.49 ± 0.56 [placebo], P = .01). Concomitantly, an insignificant decrease in reverse T 3 (rT 3) (nmol/L) was observed (0.07 ± 0.01 [GH], 0.15 ± 0.01 [placebo], P = .08). GH caused a highly significant increase in T 3/thyroxine (T 4 (×100) ratio (1.84 ± 0.12 [GH], 1.37 ± 0.06 [placebo]). Serum thyrotropin (TSH) was not significantly changed by GH. No changes in total thyroxine (TT 4) (nmol/L) (98 ± 6 [GH], 111 ± 8 [placebo], P = .40) and free thyroxine (FT 4) (pmol/L) (17.4 ± 1.3 [GH], 18.6 ± 1.1 [placebo], P = .37) after 14 days of GH administration were observed. In conclusion, 2 weeks of GH administration increases EE and lipidoxidation. This finding may partly be mediated by an increase in peripheral T 4 to T 3 conversion.